1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Brief Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “canning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to create the desired pattern of illumination on the substrate the lithographic apparatus passes the beam along a path in which various optical elements are located. Most of these elements comprise lenses that serve to focus the beam and to shape its properties. In modern equipment, which uses light of extremely short wavelength, the lenses are mostly realized as mirrors. Other optical elements may merely serve as windows that atmospherically separate different chambers of the apparatus along the beam path, but pass the beam.
Another type of optical element used lithographic equipment is an aperture screen to partially block out light from the cross-section of the beam. Typically such an aperture screen (also referred to as aperture blade) is located in a pupil plane of the beam. Such aperture screens serve to shape the dependence of the intensity distribution of the beam on the angle of incidence on the substrate. Typically, different aperture screens, with differently sized or shaped openings are needed for different processing steps during integrated circuit manufacture, or for different types of integrated circuit devices. Accordingly, lithographic equipment has been provided with a storage unit for a plurality of aperture screens with differently shaped or sized openings and a handling unit to move selected aperture screens from the storage unit into an operational position in the cross-section of the beam.
Due to the ever decreasing feature size of integrated circuits it has been found necessary to reduce the wavelength of the light used in lithographic equipment to wavelengths in the UV range or even shorter. This has led to an increased risk of light induced chemical reactions at the surface of the optical elements in the beam path towards the substrate. In particular, gas molecules from the atmosphere that surrounds the optical elements may give rise to reaction products on these surfaces, such as oxides or hydrocarbon deposits, or material may even be desorbed from these surfaces. These reaction products can adversely affect imaging of patterns on to the substrate, due to changes in reflectivity or transmissivity of the optical elements. To minimize such problems, the atmosphere near the surface of the optical elements has to be carefully controlled to ensure that potentially noxious gases are not present in excessive amounts.
This makes it necessary to detect whether more than an acceptable amount of such gases is present in the atmosphere around the optical elements and to adjust parameters that affect the atmosphere if this is the case. For the detection of some gases, and for the detection of some extreme situations, it suffices to monitor the partial gas pressure. However, for other gases monitoring partial gas pressure is not sufficient. This is the case for example when the partial pressure for a gas species is excessively low in comparison with overall pressure around the optical elements. Similarly, it may be impossible to monitor subtle deviations from a desired state, if the deviations have no measurable short-term effects on the optical elements but affect the optical elements in the long term.